On organized group rides, a sag stop is a rest stop where there are snacks and drinks. That's it.
A sag wagon, is a support vehicle that offers mechanical assistance or a ride back to base camp if you're spent!
L8R
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger